Microbial drivers of root plasticity

Soils are highly heterogeneous and dynamic systems, experiencing a constant flow of plant root exudates and moisture fluctuations that affect nutrient distribution, soil physicochemical properties, and microbial metabolisms. Plant roots adjust their development within the soil matrix (growth, branching, root angle, and anatomical features) by integrating local environmental conditions with physiologically informed signals. These physiological responses and the adaptability of roots are commonly defined as plasticity. Although genetically determined, root plasticity is modulated by local fluctuations in water and nutrient availability, environmental changes, and soil properties as well as by the root-associated microbiota. Roots and their vicinity are colonized by taxonomically and functionally diverse microbial species. Specific members within these communities can establish chemical communication with plants via root-derived signals, thereby tapping into the plant's hormonal and developmental network, influencing root plasticity. Given that most root traits associated with plasticity have been discovered under axenic conditions, our knowledge of the multiple potential mechanisms by which members of the root-associated microbiota modulate root plastic responses is still limited. In this review, we explore the recent progress in this field and identify specific avenues for future research toward advancing molecular and ecological understanding of microbial-mediated root plasticity in terrestrial systems.